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Homogenization

Homogenization is necessary because much has happened in the world between the French and industrial revolutions, two world wars, the rise and fall of communism, and the start of the internet age. Inevitably many changes have occurred in climate monitoring practices.

As a consequence, the instruments used to measure temperature have changed, the screens to protect the sensors from the weather have changed and the surrounding of the stations has often been changed and stations have been moved in response. These non-climatic changes in temperature have to be removed as well as possible to make more accurate assessments of how much the world has warmed.

If there is a bias in the trend, statistical homogenization can reduce it. How well trend biases can be removed depends on the density of the network. In industrialised countries a large part of the bias can be removed for the last century. In developing countries and in earlier times removing biases is more difficult and a large part may remain. Because many governments unfortunately limit the exchange of climate data, the global temperature collections can also remove only part of the trend biases.

Many people only know that climatologists increase the land surface temperature trend, but do not know that they also reduce the ocean surface trend and that the net effect is a reduction of global warming. This does not fit to well to the conspiracy theories of the mitigation sceptics.

Raw climate records contain changes due to non-climatic factors, such as relocations of stations or changes in instrumentation. This post introduces an article that tested how well such non-climatic factors can be removed.